The present disclosure relates to nitride field effect transistors applicable to power transistors used for power supply circuits of consumer appliances such as air conditioners and light controllers, for example.
Nitride semiconductors are large in bandgap, breakdown electric field, and electron saturated drift velocity compared with Si, GaAs, etc. Also, in an AlGaN/GaN heterostructure formed on a substrate having (0001) plane as its principal surface, two-dimensional electron gas (also expressed as “2DEG”) is generated at its heterointerface due to spontaneous polarization and piezoelectric polarization, whereby a sheet carrier concentration of the order of 1×1013 cm−2 is obtained even without doping of any impurity. A high electron mobility transistor (HEMT) using this high-concentration 2DEG as a carrier has recently been attracting attention, and various HEMT structures have been proposed (see S. C. Binari, W. Kruppa, H. B. Dietrich, G. Kelner, A. E. Wickenden, and J. A. Freitas Jr., “Fabrication and Characterization of GaN FETs,” Solid State Electronics 41 (1997) 1549-1554).
FIG. 12 is a cross-sectional view showing a semiconductor device including a conventional HEMT having an AlGaN/GaN heterostructure. In the conventional field effect transistor shown in FIG. 12, a low-temperature AlN buffer layer 2, an undoped GaN layer 3, and an undoped AlGaN layer 4 are formed in this order on an Si Substrate 1, and a source electrode 5 and a drain electrode 6 are formed to be in contact with the undoped AlGaN layer 4. A gate electrode 7 is formed between the source electrode 5 and the drain electrode 6.
With the above-described structure, two-dimensional electron gas is generated near the interface between the undoped GaN layer 3 and the undoped AlGaN layer 4 due to spontaneous polarization and piezoelectric polarization, and this gas is used as a carrier. When a voltage is applied between the source and the drain, electrons in the channel flow from the source electrode 5 toward the drain electrode 6. At this time, by controlling a voltage applied to the gate electrode 7 to change the thickness of a depletion layer right under the gate electrode 7, the drain current can be controlled.